1. Field of the Invention
The present invention relates to a boot for a universal joint such as a constant velocity universal joint of an automobile.
2. Description of the Prior Art
Universal joints are protected by rubber boots that prevent ingress of dusts and dirt. Rubber boots also serve to confine a lubricant such as grease applied to the moving parts of the universal joints.
As is well-known, the universal joint boot is alternately subjected to expansion and contraction for each revolution of the universal joint because the two axes of the universal joint are intersected at an angle with one another. In addition, the boot is subjected to an inner air pressure resulting from the temperature rise as well as to a substantial centrifugal force resulting from the rotation of the boot. Often, a high and uneven centrifugal force is exerted on the boot due to the presence of the mass of grease adhering to the inner wall of the boot. Under high revolutionary speeds, the boot is expanded radially outwardly by the increased centrifugal force. If the radial expansion is excessive, there is a danger of the boot being damaged or broken due to contact with the ground or with other parts of the automobile.
U.S. Pat. No. 4,278,262 to Mizutani et al. discloses a rubber boot wherein the wall thickness of a valley portion, and the portions adjacent thereto, of the bellows structure is increased in order to restrain the radial expansion of the boot.
While this design is advantageous in reducing the undesirable radial expansion of the bellows structure, the problem is that the flexibility of the valley portion is sacrificed so that a stress is concentrated at the valley portion when the boot is subjected to repeated cycles of expansion and contraction. This gives rise to the formation of cracks and fissures at the valley portion thereby resulting in a premature breakage of the boot.
The stress concentration is particularly significant when the boot is made of a thermoplastic elastomer material in order to improve the weather resistance of the boot, because a thermoplastic elastomer material has much less elasticity than a rubber material.
Accordingly, an object of the present invention is to provide an improved boot for a universal joint.
Another object of the invention is to provide a universal joint boot which has a high degree of ability to withstand the centrifugal force and which is yet capable of avoiding the concentration of stress at the valley portions of the boot.
A still another object of the invention is to provide a universal joint boot which is capable of restraining the radial expansion of the boot due to the centrifugal force and which has an improved durability and a prolonged service life.
A further object of the invention is to provide a universal joint boot which has an improved weather resistance and which is capable of being manufactured at a low cost.
Another object of the invention is to provide a universal joint boot which has a controlled wall thickness and which is suitable for manufacture by blow molding of a thermoplastic elastomer material.